1. Field of the Invention
This invention relates to a camera which is advantageously adapted to a camera system having an automatic focusing (hereinafter referred to as AF) device.
2. Description of the Related Art
The technology in the field of picture taking apparatuses such as cameras and others has recently advanced to a conspicuous extent. As a result, the video cameras and the like have come to be equipped with automatic focusing devices, automatic exposure control devices, etc., in general.
FIG. 1 of the accompanying drawings shows in a block diagram and by way of example the arrangement of a so-called passive type automatic focusing (AF) device. The passive type AF device is arranged to detect the focused state of an optical system from an image signal obtained by photo-electric converting an object's image which is formed through the optical system.
Referring to FIG. 1, a first lens group 101 is provided for focus adjustment. A second lens group 102 is provided for zooming. A third lens group 103 is provided for correcting a change in a focal point resulting from zooming. A fourth lens group 105 serves as an image forming system. The illustration includes an image sensor 120; an amplifier 121 which is arranged to amplify a signal output from the image sensor 120; a high-pass filter 122; and a microcomputer 123 (hereinafter referred to as AF microcomputer) which is arranged to perform AF control and to produce instructions for driving a motor. A driver 110 is arranged to supply a motor 107 with energy in accordance with the instruction of the AF microcomputer 123. The motor 107 is arranged to shift the position of the focusing lens group 101 under the control of the driver 110. A transmission line 1201 is provided for transmitting the instruction of the AF microcomputer 123. A boundary line 1202 indicates a boundary between a camera part and a lens part.
In the passive type AF system which is arranged as shown in FIG. 1, an image which has passed through an optical system is first converted into an electrical image signal by the image sensor 120. The electrical signal is amplified by the amplifier 121. The amplified image (or video) signal is supplied to the high-pass filter 122. At the high-pass filter 122, a high-frequency component of the image signal which is related to focusing within the signal is alone taken out. The high-frequency component thus taken out is supplied to the AF microcomputer 123. It is known that the high-frequency component of an image or video signal generally increases its level accordingly as the optical system is adjusted closer to an in-focus position. The relation of the high-frequency component to the focusing lens position of the optical system is as shown in FIG. 2. Referring to FIG. 2, the level of the high-frequency component of the video signal which is shown on the axis of ordinate changes accordingly as the position of the focusing lens (shown on the axis of abscissa) is shifted. The high-frequency component reaches a maximum level when the lens reaches an in-focus position for a distance to an object. Therefore, for an AF action, the AF microcomputer 123 is arranged to apply a lens driving instruction to the driver 110 via the transmission line 1201 in such a way as to cause the high-pass filter 122 to produce its output at a maximum level. In accordance with the instruction from the AF microcomputer 123, the driver 110 generates a sufficient amount of energy for driving the motor 107 to rotate.
The above-stated arrangement of the AF system necessitates the AF microcomputer 123 to form a driving signal wave form for driving the motor 107. The motor driving signal can be formed by various methods including: A method of controlling the speed of the motor by adjusting a voltage to be applied to the motor; and another method wherein the motor speed is controlled with a signal of a periodic driving wave form, such as a duty driving wave form for a DC motor, by exciting the motor only for a period during which the signal is in an on-state. However, the former method necessitates the applied voltage to be lowered in the event of a low speed. It is therefore impossible to obtain a sufficient torque at a low speed. In view of this, the latter method of performing the duty control is employed in general. In the case of the duty control driving method, since the motor control is performed within an AF control program, the period of the above-stated driving wave form is in synchronism with the program repeating period of the AF microcomputer 123. Therefore, the AF system using this driving signal wave form has been employed for most of the apparatus of varied kinds.
There has recently been proposed many interchangeable lens systems that permit interchange of lenses by separating the lens part from the camera part at the boundary 1202 while leaving the basic arrangement of the AF system unchanged from the state as shown in FIG. 1.
It is necessary, for smooth introduction of the interchangeable lens system of a video camera into the field of consumer appliances, to arrange the lenses to be interchangeable without causing any inconvenience in terms of operation. In the field of consumer appliances, the camera must be arranged to automatically perform aperture control and focus control. Therefore, the interchangeable lens system must be also arranged to cause no impediment to the automatic operation.
As regards a difference between the TV systems adopted in different areas of the world, a lens sold in the area of the NTSC system can be used without any problem in combination with a video camera of the PAL system as the interchangeable lens system of a single-lens reflex camera is irrelevant to the TV system. A converse combination also presents no problem. Therefore, the interchangeable lens system of a video camera is preferably arranged to be interchangeable with that of a video camera conforming to a different TV system for saving the users from being confused.
Generally, the apparatuses of this kind are arranged to control the lens according to control information obtained from the camera body. For this purpose, the video camera is also arranged to control the AF motor driving action in a cycle of field periods or in a cycle which is an integer times as much as the field cycle.
However, the above-stated system presents the following problem: FIG. 3 shows the vertical synchronizing (hereinafter abbreviated as sync) signal of a video signal which is obtained with the program repeating period of the AF microcomputer 123 arranged to be equal to the vertical sync period (50 Hz) of the PAL system which is one of known TV systems and the period of the driving signal wave form to be equal to the program repeating period of the AF microcomputer 123. The vertical sync signal of the video signal is shown in relation to the change-over timing of the motor driving wave form of the conventional system described in the foregoing. The illustration includes the vertical sync signal 201 which is included in the video signal of the PAL system; the vertical sync signal 203 of the NTSC system; and the motor driving signal wave form 1401.
As apparent from FIG. 3, the motor driving signal 1401 is in a pulse wave form which is formed in synchronism with the vertical sync signal 201 of the PAL system. As mentioned in the foregoing description of the conventional system, the period of the motor driving signal 1401 is governed by the program repeating period. Therefore, if the algorithm of forming the driving signal wave form is used as it is for the vertical sync signal of the NTSC system, the motor driving signal would be synchronized with the vertical sync signal 203 of the NTSC system. In other words, the signal comes to have a shorter period and shorter ON time than the motor driving signal 1401 which is in synchronism with the vertical sync signal of the PAL system. The period and the ON time of the motor driving signal wave form should be determined according to the characteristic of the motor 107 which is disposed within the lens unit. Therefore, if this driving wave form is formed by the AF microcomputer 123 which is disposed on the side of the camera body in the interchangeable lens system, the possible kinds of interchangeable lens is not only limited by the TV system but also necessitates the motor to be designed to ensure an accurate driving action which is apposite to the period and ON time of every signal produced on the side of the camera body.
Further, it would be also impossible to obtain a correct wave form unless the AF program repeating period is limited. As a result, the design latitude for the system as a whole would be greatly impaired. The freedom of choosing any desired combination which is the basic purpose of the interchangeable lenses would be denied. Besides, it would greatly increase the cost of the system.
There is no interchangeability between different TV systems in respect of signal processing. More specifically, the camera of the NTSC system, that of the PAL system and that of the SECAM system have no interchangeability in terms of signal processing as they differ in field frequency from each other. In the present situation, therefore, both the lens and the camera must be arranged to match the local TV system. This problem has made it difficult for both the users and the manufacturers to smoothly introduce an interchangeable lens system.